FUSE: require valid file/dir handle; drop open_files fallback

Several FUSE ops fell back to looking up a FileInfo via
state.open_files when the kernel-provided fh was zero, returning
-ENOENT if the lookup failed. This path papered over handle-tracking
bugs and added cost on every call. Use the handle directly and
return -EBADF when it is invalid - the kernel should always hand us
a valid fh for an open resource.

Also simplify FileInfo::from_fh / DirInfo::from_fh and their to_fh
helpers: callers now handle the null-pointer case explicitly so the
ASSERT_NOT_NULL checks are redundant.

Covers fallocate, fchmod, fchown, fgetattr, flock, ftruncate,
futimens, read, release, releasedir, write, statx, ioctl (file +
dir), readdir (sequential, concurrent-on-read,
concurrent-on-search), and the passthrough fast paths in
create/open.

src/fuse_readdir.cpp

@@ -84,6 +84,9 @@ _handle_ENOENT(const fuse_file_info_t *ffi_,
   dirent de;
   DirInfo *di = DirInfo::from_fh(ffi_->fh);
 
+  if(not di)
+    return -EBADF;
+
   if(!di->fusepath.empty())
     return -ENOENT;